Jinhyung Kim and Joshua Hicks first analysed data collected from 559 US lawyers. In 1984, the law men and women rated their life satisfaction and reported whether they had any children, and then in 1990 they were contacted again and said how many kids they now had. Lawyers who were happier in 1984 had more children in 1990, even after accounting for their income, age, gender and number of children when they were first contacted.

Of course lawyers are not entirely like the rest of us, so a more valid follow-up study was needed. This time the researchers analysed data collected from nearly 5000 people across the US in 1995-96 and then again between 2004 and 2006. Once more the data showed that people who reported more happiness at the first time point tended to have more children at the second time point.

This second survey also had the advantage that it looked at different forms of happiness. Life satisfaction, more positive emotions, and more purpose and meaning in life were all independently associated with having more children, even after accounting for other factors like income, age and gender.

Kim and Hicks said: "The current studies suggest that children may not only serve as a source of happiness, but happiness itself is linked to future reproduction." The routes by which happiness might encourage procreation remain unknown and are likely manifold and complex. Speculating about the role of hedonic happiness specifically, the researchers said: "... people in a positive affective state may use the feeling as information that they are currently satisfied, motivating them to explore new opportunities such as childrearing."

It's also likely that relationship status plays a big part in the link between happiness and having more children. Happier people are more likely to form new, and sustain existing, relationships, which obviously makes it easier to have kids. Indeed, in the second survey, the statistical link between past happiness and future children disappeared once relationship status at the second time point was taken into account.

Another detail: the relationship between past happiness and later number of children was weaker for people who already had at least one child. The researchers wondered if this is because "... realistic issues associated with parenting override the effect of cognitive well-being and optimism on the willingness to have additional children."

It's important to note that we are talking about subtle associations here. For instance, in the second survey, life satisfaction at the first time point only explained 0.001 per cent of the variance in number of children at the second time point. This might sound derisory, but remember this was after taking into account other powerful factors such as income. The researchers said the small effect sizes are to be expected "considering the number of variables that influence the probability of having a child." But to be sure, they added: "... clearly happiness does not account for all of the variance associated with parenthood."

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Kim, J., & Hicks, J. (2015). Happiness begets children? Evidence for a bi-directional link between well-being and number of children The Journal of Positive Psychology, 1-8 DOI: 10.1080/17439760.2015.1025420